I'd guess that most systems that use a full-grown ORM library have all the db-specific code in the underlying (and reusable) ORM layer, and when they do use hand-written DB specific SQL, it's only to optimize a few "interesting" JOINs).

Note that most ORM layers do rely on some standardization of the database schema (especially concerning naming of tables and columns, and limiting the available column types), but that doesn't mean the schemas themselves are simple at all.